Spacecraft attitude can be controlled by transferring angular momentum to the craft through reaction wheels or control moment gyros. Typically, accuracy is compromised because conventional ball and gas bearings presently used to suspend these devices are unable to precisely maintain the orientation of the device's angular momentum with respect to the craft.
All conventional bearing systems, including magnetic bearings, suffer further limitations when used in large spacecraft. When transferring angular momentum from a rotating wheel to a spacecraft of modest size, such as a satellite, an angular tilt of 1.degree. or 2.degree. is usually sufficient. But when larger craft such as a space station is involved, much larger angles of typically 10 or 20 degrees are required to permit greater transfer of angular momentum.
Presently, one or two degree-of-freedom gimbals are added to control moment gyros to permit the required degree of tilt. Gimbal systems, however, add weight, complexity, and cost to the attitude control system.